There are several issues in the construction process of the low-voltage windings of toroidal distribution-grade transformers that need to be resolved before mass production can be embarked. A major problem still unresolved is the lack of technology to wind thick wires. For example, the winding machines available are only capable of winding magnet wires with gauge of up to #6 AWG. The other issue is that thicker wires lose flexibility as their thickness increases making it impossible to properly bend the wires at the edges of the core. This yields to undesired inhomogeneous spaces between core and windings and also different winding layers. Therefore, more insulation is needed between the windings, the final size of the transformer increases, and the thermal performance is negatively affected.
An alternative, is to use thick stranded welding cables. These cables are flexible enough for this purpose. There are three known major problems with this winding strategy:                Currently, there is not winding machine that can handle the entire wire of the low voltage winding. This is so because, first, the cable should be completely loaded on the magazine (cable cannot be cut into pieces). The weight and the length of cable for a distribution class transformer are outside the limit of existing winding machines.        According to the above mentioned issues, the winding process is not automated. Therefore, this method it is very time consuming, labor intensive, and expensive.        Transformers designed for lower temperatures are bulky and therefore, more expensive. According to IEEE/ANSI standards dry-type transformers can be designed for 150° C. (hot spot temperature). Welding cables offer a temperature rating of up to 105° C. Operating temperatures higher than 105° C. are possible in dry-type transformers (up to 220° C.), but the insulation (jacket) of the cables is not adequate for this. Therefore, transformers need to be designed for lower temperature bringing the price up.        
The best approach is to use several thin conductors in parallel for a winding that carries large current (e.g. low voltage windings). The conventional continuous winding strategy (layer by layer), results in circulating currents which increase the winding losses tremendously. This is so because parallel windings would have different lengths, and consequently different impedances. The parallel connection of wires with different resistances yields non-uniform distribution of current between them. In this condition, the wire with lower resistance carries more current than the other conductors. This unbalanced condition produces higher losses as shown with a simple example consisting of three parallel conductors (see FIG. 1). In this figure two cases are compared: three conductors with the same length (same resistance) and three conductors with different lengths (different resistance).
The resistance of a conductor with cross section area A, length l, and electrical conductivity ρ can be computed by:
  R  =      ρ    ⁢                  ⁢          l      A      
Note that A and ρ are equal for all conductors. On the left hand case of FIG. 1, three conductors have the same length. Therefore, resistances of the three conductors are equal to R. On the right hand case of FIG. 1, the second and the third conductors are assumed to be 25% and 33.3% longer than the first conductor, respectively. The equivalent circuits of these two cases are shown in FIG. 1.
The equivalent resistance seen from the terminals of the circuit shown in FIGS. 1(a) and (b) are R/3, and 20R/51, respectively. Assuming a constant current load, the real power loss of the circuit shown in FIG. 1(b) is 17.65% higher than the real power loss of the circuit of FIG. 1(a).